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Comment by jiehong | Hacker News Reader

Kinda nice, but what I really wish is to be able to use transducers (idea from clojure) so that I can define and compose a function and then apply it to a stream / an optional / an observable, etc.

Or, at the very least, being able to define functions taking a stream<A> and producing a stream and compose it with another function taking a stream as input and apply it all to a stream<A>.

Both are just not possible with the current Java stream API.

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What do you mean by: function taking a stream as input and apply it all to a stream<A>

He means to combine them. So when you have a function A -> B and some other function B -> X, he wants to be able to make a composed function A -> X out of them.

This is a common thing in many functional style programming languages.

It's not the most convenient syntactically, as per usual in Java land, but java.utils's Function<T, R> has both a compose() and an andThen().

There is a set of ‘compose’ operators in pidove that can not only compose functions but also let you put a bifunction or supplier at the beginning of the chain or put a consumer at the end.

For it to really shine I need to add more versions of the compose function with different lengths, but I am working on a code generator that will be able automatically generate those to arbitrary length.

Java can do that trivially easy for the stream case because there is only one input and one output. E.g.

    static <A, B, X> Function<A, X> compose(Function<A, B> one, Function<B, X> two) {
      return (a) -> {
        return two.apply(one.apply(a));
      }
    }

Java sucks when there are a variable number of generic arguments, so it struggles with the composition of two functions that have a variable number of inputs. That doesn't really apply to stream processing though.

Yes, but in this case, it's f: Stream<A> -> Stream and g: Stream -> T, and compose them into h: Stream<A> -> T.

Would be nice to be able to write, for example:

T result = streamOfA().apply(f).apply(g);

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